Low frequency hydrophone



Sept. 26, 1961 D. w. KUESTER 3,002,179

LOW FREQUENCY HYDROPHONE Filed Sept. 3, 1952 2 Sheets-Sheet 1 FIG. 1.

INVENTOR K U ESTER ATTORNEYS p 1961 D. w. KUESTER 3,002,179

LOW FREQUENCY HYDROPHONE Filed Sept. 5, 1952 2 Sheets-Sheet 2 IN VENTORDONALD W. KU ESTER BY FIG.6.

ATTORNEYS United States Patent 3,002,179 LOW FREQUENCY HYDROPHONE DonaldW. Kuester, Hyattsville, Md, assignor to the United States of America asrepresented by the Secretary of the Navy Filed Sept. 3, 1952, Ser. No.307,727 11 Claims. (Cl. 340-10) (Granted under Title 35, US. Code(1952), see. 266) The invention described herein may be manufactured andused by or for the Government of the United States of America forgovernmental purposes without payment of any royalties thereon ortherefor.

The invention relates to an underwater transducing device and moreparticularly to a structure embodying a crystal of the barium titanatepiezoelectric ceramic variety, compressional wave influence stressingstructure and a new and novel manner of mounting and loading the same.

The instant invention is particularly adapted for low frequencyunderwater signalling and detection at frequencies in the range of 5 to400 cycles per second and provides increased sensitivity characteristicsover prior art underwater crystal type transducers.

Both the improved crystal mounting arrangement of this application andthat of the copending application, supra, incorporate an assembly of apair of disc configured centrally loaded Bimorph ceramiccrystals, and aSylphon bellows mounted at the periphery of the crystal plane for thepurpose of masking the interior surfaces of the oil immersed Bimorphdiscs from signal energy. The term Bimorph as employed herein may bedefined as indicating that two crystal elements are cemented together.

Although the prior type structure incorporates a relatively largeBimorph disc while the improved type incorporates a smaller disc bothversions are subjected to bending moment loading under signal energyconditions to drive the crystals to a voltage output relationship undercompressional and tensional stressing as hereinafter described ingreater detail.

In hydrophones incorporating the large diameter Bimorph crystal discs ithas been observed that the voltage increments developed across thecrystal structure at the outer periphery thereof are of less magnitudethan that of the voltage increments produced at the central portion ofthe crystal. The reason is apparent if a diametric cross section of thecrystal is considered as a double cantilever beam wherein the outerextremity of the beam or the peripheral areas when taken as a disc, aresubjected to less bending moment-stressing and as a consequence thereof,less tensional stressing in the upper crystal of the Bimorph andcompression in the lower crystal under positive signal load conditions.This relationship will be apparent when consideration is given to therelationship wherein a radial section across half of the disc is takenfor analysis and considered as a cantilever beam with the supportthereof at thecenter and a uniformly distributed load applied thereto bythe input signal energy. This reduced voltage output at the outer edgeor zone of the crystals resulting from bending moment effects isparticularly evident when this output voltage effect occurring withtensional stressing in the upper crystal of the Bimorph andcompressional Wave loading of the transducer segments in the lowercrystal of the two under a uniformly distributed signal lead is comparedwith that taken at the central zone portion thereof. This stressingunder consideration is that taken in the plane of the crystal ratherthan the compressional effects which appear across the thicknessthereof.

In the prior type hydrophone it appeared desirable to ICC g connect theseveral electroded pairs of crystal elements of the transducers in amanner effecting a series relationship to provide an additive voltageoutput therefrom. However, it has been observed that in so doing, thesegment configured crystal element electroded areas which extend to theouter peripheral edges, when considered on an incremental area basis,are in effect a group of voltage averaged increments appearingelectrically as a voltage generator and a series capacitance, andconnected in shunt with a similar generator and series capacitance foreach of the adjacent incremental transducers theresurrounding. Theunequally stressed parallel coupled' transducer generators at the discperiphery are subjectedto less force and consequently less tensional andcompressional voltage producing efliects than that of the transducerincrement near the center and therefore produce considerably loweroutput voltage.

This effect is particularly evident when the transducer segmentscomprise, respectively, a first or inner circular. electroded area and aseries of annular configurations in concentric relationship. The use ofconcentric segments for test purposes provides a means of comparison ofthe actual voltage output at various radial distances from the center.It has been determined by these tests that the additional capacitanceresulting from the electroded area at the periphery produces increasedvoltage averaging and less total voltage output than that of a smallercen'- trally loaded crystal if driven by an annular metallic stressingmember of the character of the instant invention and in an arrangementas herein embodied. This smaller diameter Bimorph transducer as drivenby signal responsive arrangements of the hydrophone produces anincreased total output voltage when compared to that of a larger crystalwith less voltage averaging effect. It also provides resultant economyin crystal material and greater ease of production. 7 F

This improved device in essence overcomes the disad-' vantages of outputaveraging effects of electroded segment areas at the peripheralstressing zones which are apparent when consideration is taken withrespect to the relationship as hereinabove described and occurringacross either a group of annular segments or a radial extensive seg;

' ment. The coupling of incremental transducive elements in bothinstances is such as to produce a shunt capacitance resulting fromparallel electroded dielectric material and connects the same inparallel with each of the incremen tal crystals generative outputs. Thisshunting or averag ing efiect is not to be confused with the seriescapacitance existing due to distributed capacitive effects when seg-fmented units are series connected to provide a voltage additive outputfrom the entire series of segmental elements. 1

The instant invention is directed to a ceramic crystal type voltagegenerator of relatively small diameter, and a driver therefor providingbending type loading for the barium titanate piezoelectric material of adisc type Bimorph in a manner to produce an equivalent satisfactory.voltage output therefrom with reduced capacitive characteristics overthat of prior type large crystal units. Moreover, the instant inventionobviates manufacturing problems involved in producing a crystal ofcomparative ly large diameter which requires special manufacturing;polarizing and handling techniques to prevent breakage thereof and toinsure the desired piezoelectric character! istics. In the presentinvention the crystal produces greater output than that of largercrystal units, with aconsiderably reduced crystal size and additionallypro vides for crystal loading of the same order whereby the- Bimorphelement and driver therefor are adapted for use in the same or a similarmounting structure environment in a hydrophone transducer.

Imaccordance with the foregoing, one feature of the instant inventionresides in the provision of a highly sensitive underwater transducerincorporating a Bimorph ceramic type crystal element of reduced diameterover prior devices, and the provision of a driver element thereion'iorimproved. crystal response wherein all the advantages of prior artciystal loading devices are obtained with a more economical utilizationof ceramic material and further whereinmany of the disadvantages of suchdevices heretofore or now in general use are obviated.

One. object of the instant inventionresides in the in corporation of aplurality of signal sensitive driver elements for a plurality of disctype Bimorph crystal elements wherein improved crystal driving action isprovided in conjunction with advantageous central loading thereof.

Another object lies in the advantageous signal stressing ofzBimorph disctype ceramic transducer crystals with reduced capacitive averagingeffects appearing in the voltage output therefrom.

An additional object resides in the provision ofirn proved utilizationof barium titanate ceramic type Bimorph transducer crystals therebyproviding higher sensitivity and output characteristics in low frequencyapplications.

It is also an. object of this invention. to provide a transducer whichis highly sensitive to low frequency signal intelligence in whichtransient peak signals of high intensity are limited by an overloaddevice thereof and which functions in conjunction with the driver .forthe crystal element thereof to prevent damage thereto.

Other objects and many .of the attendant advantages of ,this inventionwill be readily appreciated as the. same becomes better understood byreference to the following detailed description when considered inconnection with the accompanying drawings wherein:.

FIG.'1 is an elevational view in vertical sectionshowing. the generalconstruction .of .the hydrophone, transducer and details of the annulardrivers for thecerarnic crystal elements;

FIG. 2 is a plan view of the hydrophone of BIG. 1;

FIG. 3 is an illustration in plan showinga single .elece troded crystalelement;

FIG. 4 is a plan view of a ceramic discshowinga plurality of segmentallyelectroded areas thereon;

FIG. 5 is a diagrammatic view illustrating the signal loaded conditionsof the pair of Bimorph crystals in a transducer assembly and acomparison of the developed voltage polarities under loading with thepolarities at assembly under static conditions, and;

FIG. 6 is a diagrammatic illustration showing the mannor of polarizationand connection .of a segmented Bnnorph crystal of FIG. 4.

Referring now more particularly to FIG. 1 of the drawings wherein theshowing is of a crystal mounting and loading structure of a preferredembodiment of the mounting arrangement therefor, the hydrophonegenerally indicated by reference character 1 comprises a support or base2 to which is mounted a housing member 3 which provides a chamber 4therein for reception .of the crystal assembly generally indicated at 5.

The hydrophone assembly 1 is described with respect to a multi-segmenttransducer arrangement of one an bodiment thereof as shown in FIG. 4. Itis to be understood, however, that the continuous electrode surfacedcrystal of FIG. 3 discloses structure in accordance with a preferredembodiment of the invention.

The transducer crystal mounting arrangement for either type of electrodeconfiguration comprises a set of mounting bosses 6 which are integralwith the housing to provide support for the Sylphon bellows element 7.The bosses are preferably provided on the interior of the housingassembly in order that the crystal assembly may be mounted within thechamber-with a fluid medipm 16 there surrounding and in sealedrelationship with respect to the exterior housing. The chamber 4 isprovided with a filling hole 8 in the bottom for the introduction offluid to the interior thereof. The cap 9 functions to close this holeafter the filling of the chamber with suitable fluid 16 such forexample, as silicone .oilor other liquid having an acoustic impedancesimilar to that of sea water.: The cap 9' is inserted after all free orentrapped air or other compressible gas, other than that sealed withinthe gas bag 29 has been eliminated. Suitable seal between the housingand the cap is provided by the gasket or .O-ring member 10 which iscarried thereby.

The housing member .3 is attached. byany suitable means, such forexample as bolts '11 to the base member 2, and is adapted at the upperextremity thereof for mat-. ing engagement with a formed rubber coverelement12. The cover element is protected by a guard member 13 whichmaintains an assembled relationship of the cover 12 and housing member 3by screws 14 or other suitable attachment arrangements. The guard member13 is pro: videdwith suitable apertures at 15 for fluid communica.-.tion between the exterior thereof and the rubber cover. The interior of.the housing as enclosed by the rubber coveris .adaptedior the. receptionof hydraulic fluid .16 which provides, signal .comnrunicationbetween therubber cover element .12mand. the exterior surfaces of the crystalassembly indicated generally at 5. The crystal assem: bly ismaintainedina fixed position by a screw mounted spider assembly '17.which is attached to the boss elements 6 provided in the housing'andrtowhich the bellows assembly is centrally..afiixedsininterposed relationbetweenthe pair of crystal driver elements; 18.. The bariumtitanatecircular Bimorph crystals 19..are;attached to the. bore of the metallicdriver elementslfiby low, dielectric. constant cement 20. at the :outer.peripheral edges thereof. These elementsld. are in turnattached as by aspinning over operation, to the upper and lower extremities, of thebellows at their respective peripheral face portions. Thebellowsfunctions as a mask to preventntransfer of signal energy from thefluid-in chamber '4 to ;the;in. terior Bimorphsurfaces. It preventsvoltage Lcancella: tion eifects since it is substantially rigid tosignal influence applied thereto in a radial directionwhile beingextremely compliant to driver element movement in-an axial direc-. tion;The crystal units-19 are loaded by a suitable loading assembly '21 atthe center thereof which effects a substantially rigid loading undersignal pressure but which yields in sliding frictional relationships toa bottoming condition under. high amplitude energy SHChzflS produced bycountermining'explosions. The crystals. are thus arranged in a parallelspaced relationship with suit+ able. loading being provided by theassembly 21 which comprises a frictionally yieldable clamp 22 and ablock member .23 respectively attached to the spaced crystals 19- byscrew type fasteners-24 or other attachment arrangements. A clampingscrew 25 is interposed between slots of the block for controlling theslip of the frictional surfaces at 26 between the clamp 22 and the block23 to provide a predetermined loading at the centers of the crystalelement for signal sensitive loading operation and to permit slip underadverse loading to thereby prevent crystal damage.

The electrical connections to the crystal elements are made at terminals34'attached'to screws 24 and brought out of the casing 3 throughasealing-type insulated'thru panel terminal at 35 with the location onthe casing bottom as shown in FIG- 2. The interelectrode connections areas shown in FIG. 6. One of the crystals as illustrated in FIG. 1 isprovided with an orifice 27 for communication between the hydraulicfluid exteriorly of the crystal and the interior of the bellows system,andfunctions to filter out very low frequency changes such for example:as tide eflFects.

The interior arrangement of the bellows assembly vinaccents eludes acylindrical elcment'28 formed by the extension of the inner portion ofthe spider element 17. It is so formed as to provide a generally tubularanti-overload element which extends therefrom into spaced adjacentrelation with the inner surfaces of the pair of annular driver elements18 for the pairs of Bimorph elements 19 to prevent damage to the crystalstructures under adverse transient effects as may be encountered when anearby mine or countermining explosion occurs.

The bellows assembly additionally includes an annular gas bag unit 29which by way of example is shown in FIG. 1 as comprising an inner rubbersponge 30 and a rubber skin 31 in encasing relation therewith. This unitmay however comprise any suitable cellular mass preferably of syntheticrubber which is resistive to anydestructive effects thereon by thehydraulic fluid utilized in the unit. It is this unit which iscompressibly deformed by the fluid 16 within the bellows when thecrystals are subjected to, and deformed by, signal energy. As thecrystals are returned to the normal condition thereof or are undernegative signal wave energy effects by action of the stress relaxationin the deformed elements with reduction in signal energy intensity, thegas bag then reforms for the next succeeding positive signal cycle.

Although the drawing figures are directed to a device utilizing a pairof Bimorph units which are separated by a common bellows, and whichincorporates a common central pillar for crystal loading purposes, it iswithin the scope of this invention to utilize a single Bimorph only; inwhich case one of the Bimorph and driver units would be replaced by ametallic member attached to the central support member 22 by screws 24at the center and to the bellows in the same manner as illustrated, andwhich constitutes a circular disc rather than the annular driver andcrystal arrangement shown in FIG. 1. This structural modification is notshown since it is believed obvious to one skilled in the art. Thevoltage output derived from such a modification would be proportionatelyless than for the preferred embodiment herein described.

In the operation of the hydrophone as mounted on a mine casing in sealedrelationship therewith, the guard and cover external thereto arearranged to present the rubber cover in contact with the surroundingwater in a manner whereby the signals transmitted through the fluid arepassed by the openings in the guard structure to the effective diaphragmor cover. This rubber cover which is in fluid communicative relationshipwith the hydraulic fluid in the chamber has an acoustic impedancesubstantially equivalent to that of sea water as does the hydraulicfluid with which the hydrophone is filled. The sound energy thus imposedthrough the water to the fluid therein is transmitted in substantiallyundiminished relationship to the opposed surfaces of the polarizedcircular Bimorph discs and masked from the interior surfaces by thebellows structure.

The electrical characteristics of the barium titanate crystal ashereinbefore set forth are such as to produce voltage across theelectrodes thereof as the Bimorph disc plates are deformed byapplication of signal pressure energy thereto and provide a radiallyuniform characteristic therefrom. The voltages derived from theelectroded area or segments 32 on the ceramic discs 33 of FIGS. 3 and 4,which are initially polarized and assembled as shown in FIG. 6, and inFIG. 5 at the left edge of the Bimorph units, are effectively connectedin series relationship from crystal segment to the next adjacentelectrode segment in a manner to provide an output voltage representingthe summation of the voltage developed across the several elementsthereof with application of signals thereto. The signal load functionsto alter the polarities of the tensionally stressed element of theBimorph in a manner as shown at the center of FIG. 5. Since the outputsof all voltage generative segments are series additive the assembledrelationship as to initial,

polarity of the upper Bimorph with respect to that of the lower Bimorphis reversed.

The signal may thereby be applied simultaneously to both the upper andlower Bimorph.

The fluid contained by the casing is permitted to pass through thebleed-off orifice 27 with very low frequency pressure changes such asmay be due to tide and wave action. It provides a filter function whichis analogous to an electrical inductance with some resistance. The fluidwithin the bellows, as displaced by signal energy loading action andapplied to the drivers and the crystal units, functions to compress therubber sponge gas bag within the bellows. This gas bag displayscharacteristics comparable to an electrical capacitance. This bagthereafter reforms or discharges with the negative cycle of the signalor with return of the crystal to normal position.

It is thus apparent that a hydrophone has been disclosed which may beused for signal reception or transmission and which provides propertiesof greater sensitivity to underwater signal influence applied thereto atlow frequencies with a resultant economy in crystal size. The new andnovel driving arrangement for the crystal when combined with the centralloading arrangement and the peripherally mounted bellows suspension bywhich the crystal is substantially freely suspended provides improvedvoltage output characteristics and provides a relationship whereinadjacent segmental electrode plates may have the same polarity withoutdetrimental effects. Furthermore the distributed capacitance effectsof'series connected crystal segments is minimized by use of smallerelectrode areas than that of prior types.

The capacitive effects between the driver elements and the'highdielectric constant ceramic is minimized by utilization of a lowdielectric cement bond 20 therebetween and by suitable spacing of theelectrodes from thedriver.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:

1. A low frequency underwater transducer of the character described,comprising a pair of parallel spaced Bimorph ceramic crystal elements,axial loading means therefor centrally disposed between said pair ofBimorph crystal elements, a pair of plate type annular driving membersrespectively for each of said pair of crystal elements afiixed to theperipheral edge of said Bimorph crystal elements and extend outwardlytherefrom, and a bellows type resilient signal masking means disposedbetween said pair of driver means and arranged coaxially' with respectto said axial loading means.

2. A crystal hydrophone comprising a pair of parallel supported bariumtitanate Bimorph ceramiccrystal ele ments, a pair of annular driverplate members comprising one respectively for each of said elements andconnected to the peripheral edges thereof and extending radiallytherefrom, a resilient signal masking means extending between the outerperipheral edges respectively of said pair of driver means in concentricrelation with said pair of ceramic crystal elements, and a crystalloading means for said pair of crystals and connected thereto in coaxialrelationship with said masking means.

3. The hydrophone of claim 2 further characterized by a casing meanstherefor, fluid in said casing, said casing comprising a yieldable covertherefor adapted for receptive and transmissive response to signalenergy applied thereto and for passing said energy therethrough to andfrom a fluid energy coupling means in said casing which is in signalcommunication with the external surfaces of said drivers and said pairof crystals.

4. In a hydrophone of the character disclosed, a casing, a yieldablecover therefor, a Bimorph ceramic crystal microphone therein, suspensionmeans for said microphone wherebytthe same is suspended from saidcasing, fluid means for signal energycoupling between said cover andsaid microphone, said microphone comprising a pair of panel-telsupported Bimorph ceramic crystal elements, a pair of annular drivermeans for said crystals and arranged respectively in concentric radiallyextending relationship thereto, an expansible bellows means peripherallyattached to-said pair of driver elements at the respective ends 'of'thebellows and supported by said supporting means in the casing, and anaxial loading means for said pair of crystals which is yieldable underexcess loading conditions.

5. "The: structure of claim 4, wherein each of the crystal elementscomprises a Bi-morph structure having electroded areas oppositelydisposed and in concentric relation with the crystal Bimorph.

6. A claim according to claim in which the electroded areas areelectrically connected in a series additive voltage output relationship.

7. The structure of claim 6 wherein the last named means in saidmicrophone comprises a sealed annular gas'ba g element which is disposedabout said axial loading means-and between said pair of crystals.

8. The structure of claim 4, wherein the crystal elements comprise aplurality of segmented pairs of electroded'areas, electrically connectedin a series additive voltage output relationship in a manner to providefor signal reception thereby.

9. The structure of claim 4 further characterized by the inclusion ofsaid microphone of anti-overload means supported by and connected tosaid support and extending in transverse spaced 'adiacently to saiddriver elements and in concentric adjacency to the inner surfaces ofsaid bellows, and means interiorly of said anti-overload meansandconcentric with loading means for yieldably maintaining said fluidmeans disposed about said crystals and therebypositioning. saidBinrorplrnormally out of contact. with said overload meansavhereby thecrystals. are aided.

between said crystals, and a hydraulic fluid containedrby,

said casing for couplingisignal energy passed by said casing to the pairoflouter crystal surfaces.

ll. A Bimorph ceramic. crystal hydrophonecomprising.

a casing, a pair of parallel disposed disc configured Bimorph ceramiccrystals in said casing, a pair of annular. driver elements respectivelyfor each of said crystals and. connected by low dielectric constantcement in coplanar relationship respectively to the outer peripheraledges of each of said crystals, 21 bellows type supporting meanscoaxially arranged. with respect to said pair of drivers and at theouter periphery thereof, and a yieldable loading device disposedcentrally between the crystals, saidloading device comprising a receiverelement, a plunger element frictionally received by said receiverelement andcontrol means for maintaining a substantially rigid rela-vtionship between the plunger and receiver elements under normal signalintensity conditions and for permitting yieldable movement therebetweenunder overloadsignal energy conditions.

References Cited in the file of this patent UNITED STATES PATENTS2,105,010 Sawyer Janrll; 1938 2,126,436 Williams Aug. 9. 1938 2,414,489Shomer Jan. 21, 1947 2,444,620 \Vil-liams July 6, 1948 2,565,159Williams Aug. 21, 1951 2,576,155 Trent Nov; 27, 1951

